1. Field of the Invention
The present invention relates to a vapor phase growth apparatus, and more particularly to a vertical type vapor phase growth apparatus which has an unit for removing an unwanted deposit.
2. Description of the Related Art
Vapor phase growth techniques are necessary in process techniques for making semiconductor devices. These techniques have been used for providing a single-crystal semiconductor film (an epitaxial film) or an insulating film (e.g. SiO.sub.2, Si.sub.3 N.sub.4, or Al.sub.2 O.sub.3) on a substrate such as a silicon semiconductor.
Vapor phase growth apparatus are classified by heating systems of substrates, pressure states within reaction chambers during film formation, and also classified by energy sources such as plasma, ultraviolet rays, or laser beams. The apparatus may be further classified by types of reaction chambers such as lateral or vertical types. FIG. 4 is a cross-sectional view of a conventional vertical-type vapor phase growth apparatus. This apparatus is most commonly used because its throughput per unit time is satisfactory and its performance for forming uniform films is excellent. A reaction chamber 1 has an upper open-end portion, and a gas supply unit 2 for supplying a reaction gas to the reaction chamber 1 is gastightly secured to the upper open-end portion. The upper end of the gas supply unit 2 is gastightly closed by a cover 3. Reference numeral 4 represents the inside of the reaction chamber 1. A rotatable cylinder-type suscepter 5 is attached to the cover 3. When the cover 3 is arranged so as to seal the gas supply unit 2, the suscepter 5 is placed at an appropriate position within the reaction chamber 1. Fourteen semiconductor wafers 6 of e.g. silicon are mounted on the outer surface of the suscepter 5. The number of wafers which can be mounted on the suscepter 5 depends upon the size of the wafers. Heating means (not shown) is provided inside or outside the reaction chamber 1. For example, a reaction gas is heated using halogen lamps. In place of the heating means, high frequency heating may also be used. An exhaust unit 8 including a water jacket 81 is arranged under the lower portion of the reaction chamber 1. The gas supply unit 2, which is arranged on the upper portion of the reaction chamber 1, is provided with two gas supply nozzles 7 through which a reaction gas is introduced into the reaction chamber together with a carrier gas containing N.sub.2 or H.sub.2. Dichlorosilane (SiH.sub.2 Cl.sub.2), trichlorosilane (SiHCl.sub.3), silane (SiH.sub.4), and silicon tetrachloride (SiCl.sub.4) are used as the reaction gas. According to demand, diborane, arsine, stibine, phosphine, or another required substance is added to the reaction gas as an impurity material to be added to the films formed by vapor phase growth. The reaction gas introduced into the reaction chamber 1 flows along the outer surface of the suscepter due to gravitation, etc. Thermal decomposition occurs on the outer surface of the suscepter 5 in the atmosphere heated by the heating means provided outside the reaction chamber, thereby providing an epitaxial film on the wafers 6. After the thermal decomposition, the reaction gas is discharged through the exhaust unit 8 arranged under the lower portion of the reaction chamber 1.
In the vapor phase growth apparatus described above, a part of the reaction gas is solidified depending upon a change in the composition ratio or the ambient temperature, and is deposited on the lower portion of the reaction chamber 1. It may be considered that the unwanted deposit has a composition including Si, Cl, O.sub.2, etc., irregularly mixed. During film formation, the deposit is agitated and swirls upward within the reaction chamber 1, adhering to the surfaces of the wafers 6. Since the film formation process is repeated over and over, the number of particles adhering to the films increases, with the result that crystal defects, such as stacking faults, pits, etc., may occur in the films, thus preventing the formation of a high quality film, as can be seen in FIG. 3, which shows film characteristics obtained as the result of experiments after the film formation processes.
In FIG. 3, the axis of abscissa indicates the number of the film formation processes, and the axis of ordinate represents the number of particles deposited on one wafer (W) and the number (density) of crystal defects per sq. centimeter on the wafer. In the experiments, only those particles having diameters equal to or greater than 0.2 .mu.m are checked, since extremely small particles cannot be detected. Fourteen silicon wafers 6, each having a diameter of 150 mm, are mounted on the suscepter 5, and processed in one batch. The reaction gas, composed of dichlorosilane (600 ml/min) and the dopant gas (200 ml/min: phosphine PH.sub.3) of the impurity material, is introduced into the reaction chamber 1 through the gas supply nozzles 7. As can be seen in FIG. 3, the number of particles are increased only very slightly from the first processing to the fifth processing. However, after the fifth processing, the number of particles are increased considerably as the film formation process is repeated. No crystal defects are detected until the fourth processing. However, after the fourth processing, the number of crystal defects are increased considerably. Thus, the characteristics of the films are degraded as the film formation process is repeated.
Therefore, when the crystal defects and the amount of the particles are increased to an unacceptable degree, the unwanted deposit has been removed from the wafer surface, using a strong acid such as hydrogen fluoride.
As one of reasons why the deposit, which is a by-product of the reaction gas, is provided on the wafers, it may be considered that there is an effect of the electrostatic charge generated during the film formation. However, the electrostatic charge generated can be eliminated by heating the reaction chamber to 500.degree. to 700.degree. C. By eliminating the static electricity, the adhesion of the deposit to the wafers is reduced.
As can be understood from the above, a conventional vapor phase growth apparatus has the disadvantages that crystal defects in films and particles adhering to wafers are increased due to the deposit produced as a result of a part of the solidified reaction gas. The cleaning treatment for removing the deposit by means of the strong acid such as hydrogen fluoride requires that the film formation be stopped for every batch processing, thereby making continuous formation of films impossible, which inhibits improvement of the throughput. In addition, hydrogen fluoride or the like is highly corrosive, and the cleaning treatment using the same is limited in conditions. Therefore, such a cleaning treatment is not practical from an industrial point of view.